1. Field of the Disclosure
This disclosure relates to a method of measuring the attenuation of seismic waves in earth formations. An attenuation coefficient may provide information about seismic lithology and fluids in earth formations. In addition, the attenuation coefficient is used in deconvolution of seismic data, thereby providing improved imaging of the subsurface. Quantitative analysis of amplitudes is complicated by Q during amplitude variation with offset (AVO) analysis of seismic data where attenuation effects are superimposed on AVO signatures.
2. Description of the Related Art
In surface seismic exploration, energy imparted into the earth by a seismic source reflects from subsurface geophysical features and is recorded by a multiplicity of receivers. This process is repeated numerous times, using source and receiver configurations which may either form a line (2-D acquisition) or cover an area (3-D acquisition). The data which results is processed to produce an image of the reflector using a procedure known as migration.
Sediments in the earth are attenuative, i.e., they absorb seismic energy. One result of the attenuation is that the bandwidth of a propagating seismic signal deceases as the wave propagates into the subsurface. As a result of this reduced bandwidth, the resolution of seismic reflectors decreases with depth. Knowledge of the attenuation coefficient (typically expressed by a constant α in nepers/wavelength of the seismic wave) can be used to deconvolve the seismic data and improve the resolution. In addition, Q is correlated with lithology and is highly dependent on the gas saturation of sediments. Knowledge of Q may thus provide a useful indication of lithology and fluid content of earth formations.
Knowledge of Q is very desirable, yet it is rarely measured. If a well has been drilled, core/laboratory and vertical seismic profiling (VSP) methods can be used. Each method has advantages as well as limitations. Many laboratory-based and field measurements of Q and its dependence on parameters such as lithology and gas saturation have been made on core samples.
The attenuation coefficient α is conventionally estimated using measurements from a Vertical Seismic Profile (VSP), though it may also be estimated from surface seismic data. Vertical (zero-offset) VSPs or check-shot surveys are nearly ideal configurations for estimation of Q. However, even in VSP data, a conventional approach normally provides low vertical resolution and quite often low accuracy. The reason is that in the conventional approach, only a small portion of input data is used to estimate Q. That is why the question of reliable Q-estimates remains. In theory, interval Q may be estimated for all two consecutive receiver depths, but in practice this is impossible. Two consecutive depth spectra may be too similar, and the difference is often so small that the Q estimates have significant errors.
The present disclosure is directed to an improved method of estimating attenuation from VSP data.